Printing processes as a means for material deposition is an efficient way to increase material usage and to eliminate the photolithography process. One of the major challenges for printing high-resolution patterns is limiting uncontrolled spreading of ink on the substrate. In field-effect transistors, for example, a narrow channel (the gap between source and drain electrodes) is required in order to obtain a sufficient current at a low driving voltage with a high-frequency switching speed. For example, high frequency (HF) Radio-Frequency Identification (RFID) tags operate at a minimum frequency of 13.56 MHz; as a result, a printed HF RFID requires a channel length of 1 to 5 μm. Conventional printing methods can only reliably produce electrodes with a minimum gap of 10 μm to 50 μm, in order to avoid an electrical short between the two electrodes.
Photolithography is a well-established method for the microfabrication of thin film patterns, but it is a relatively high cost and complex method. A combination of photolithography and printing processes has been demonstrated for the fabrication of inkjet-printed organic thin film transistors (OTFTs). Sirringhaus et al. demonstrated a printed channel length of 5 μm obtained using a photolithography pre-patterning process. See Sirringhaus, H., Kawase, T., Friend, R. H., Shimoda, T., Inbasekaran, M., Wu, W., and Woo, E. P. Science 290, 2123 (2000). The gap between two printed electrodes is defined by a patterned polyimide strip, which acts as a barrier between electrodes and thus defines the channel length. FIG. 1 shows a photolithography technique of the prior art.
Microcontact printing, using an elastomeric stamp as shown in FIG. 2, is another method to pre-pattern the ink barrier strips before printing conductive materials. Rogers et al. reported a 2 μm channel length by microcontact printing. See Rogers, J.-A., Bao, Z., Makhija, A., and Braun, P. Adv. Mater. 11, 741 (1999). However, the method is based on soft lithography, which involves the replication of a stamp from a master fabricated by conventional lithography and etching.
A lithography-free self-aligned method to fabricate drain and source electrodes with a narrow gap was proposed by Sirringhaus et al. in 2005. See Christophe W. Sele, Timothy von Werne, Richard H. Friend, and Henning Sirringhaus, Adv. Mater. 17, 997 (2005). It uses a carbon tetrafluoride (CF4) plasma treatment to form a thin layer of fluorinated layer on the first printed PEDOT:PSS electrode; as a result, a high surface energy contrast exists between the fluorinated PEDOT:PSS and the substrate, and the second printed PEDOT:PSS droplets will flow off to the substrate as shown in FIG. 3.
As shown in FIG. 4, a patterning method based on the control of surface energy through a UV irradiation process with a photomask has been demonstrated by K. Suzuki et al. (K. Suzuki, K. Yutani, M. Nakashima, A. Onodera, S. Mizukami, M. Kato, T. Tano, H. Tomono, M. Yanagisawa and K. Kameyama, International Symposium on Electronic Paper 2010; http://www.ricoh.com/about/company/technology/tech/pdf/idw10paper.pdf) and achieved a minimum gap of 2 μm between two electrodes.
Tomoyuki Yokota et al. reported an electrostatic inkjet printing head using 0.5 femtolitter nozzle, which can obtain a printed Ag line width of 1 μm and a channel length of 1 μm. See Tomoyuki Yokota et al. MRS Communications 1, 3-6, 2011 and http://www.sijtechnology.com/en/super_fine_inkjet/index.html.
Rogers also proposed electrohydrodynamic jet printing, in which a drop of a conductive ink is sharpened by an electrostatic field. The method can produce lines down to 1 μm in width. See also Park, J.-U., Hardy, M., Kang, S. J., et al., High-resolution electrohydrodynamic jet printing. Nat Mater. 6(10), 782-789 (2007).
It is believed that commercial printing technologies can only achieve a printed gap around 10 μm using the surface energy engineering technique. See W. Tang, Y. Chen, J. Zhao, S. Chen, L. Feng, X. Guo, IEEE NEMS, p. 1171 (2013).
There are many advantages of using a photolithography process for microfabrication patterning, which include high resolution, speed and parallel patterning capabilities, reproducibility, etc. However, the process complexity and incompatibility between the materials and photoresists, solvents and developers are the main challenges for low-cost printable electronics fabrication on flexible substrates. A straightforward method to reduce cost is to develop a direct, printable microfabrication patterning process to eliminate the photolithography.
The self-alignment method proposed by Sirringhaus achieved a submicrometer channel length without photolithography. However, the fluorination of the first electrode alters its physical properties and those of the resulting device; the variations in gap sizes are too large to produce any useful circuit, as seen in FIG. 3.
The Suzuki method to control surface energy is efficient, but a photomask is still required, which limits the substrate size and flexibility of pattern design, and increases the cost.
The electrostatic print head with femtoliter droplet has very limited printing speed and only a single nozzle can be used to date.
Electrohydrodynamic jet printing produces the finest lines but the printing is very slow, and charged ink drops may present problems in devices. The electrostatic and electrohydrodynamic techniques seek to obtain fine patterns by reducing the size of the drop being jetted, as shown in FIG. 5, but do nothing to control its spreading on the substrate.
Accordingly, a new or improved printing technique would be highly desirable in order to facilitate fabrication of printable electronic devices.